One of the factors never investigated in these types of studies of cognition is there is no baseline per se...the one study cited stated 3.8K 7th graders were followed for 4 years and their cognition showed decline.
Sorry, have an adult in an authority position [teacher] ask a 13ish adolescent if they smoke illegal pot they will answer NO to stay out of potential trouble. Now next year the same batch or new study participants have figured out what the study's about and youthful bravado sets in ~ of course we use pot!
Finally, to assure reliability the principals conducting the study use exactly the same computerized test on the participants who are now bored taking this stupid test for the third/fourth time so they become lackadaisical in their responses [read they don’t give a crap] which shows significant decline in their cognition results.
Tests were conducted ‘12 for 4 years, uh ‘16 results ready and released in ‘19 after legalization....yepper newspeek at it’s finest!!
Uh nightmare, that’s stupid scientific research anywhere.
[sidebar: nobody ever asks or delves into “why” youth [or adults] feel the need to self medicate from life’s adventure(s)]